The present invention relates to a process including the use of a horizontal rotary furnace for the continuous manufacture of solid alkaline silicates, and more concretely speaking the object of the present invention is to provide a process for operating a rotary furnace of the above-cited type provided with an improved discharging device for the resulting product.
Generally, the obtention of alkaline silicates is based on the fusing of quartz sand (silica) with the carbonate of the corresponding alkaline cation, it being feasible to effect the melting in Bassin furnaces (shaft type), in crucible furnaces, or, in modern times, by means of a continuous process using the above-cited tubular rotary furnaces.
These rotary furnaces consist of a metallic tubular body which is interiorly lined with a refractory material, the length of this tubular body being from 6 to 8 times as great as the interior diameter of the same, and it rotates slowly with a slight rake so that the starting materials are being charged through its higher end and advance owing to the rotation and rake of the furnace and are discharged through the lower end in which, in addition, the burner of the furnace or the heater is arranged.
In the specification hereinafter set forth reference is made preferably to the manufacture of the silicate whose cation is sodium, for the process in itself is similar to that for the remaining silicates (potassium silicate, lithium silicate, etc.), the difference lying solely in the melting temperatures which are fixed by the corresponding binary diagrams and by the molar relationship existing between the silicium anhydride and the oxide of the corresponding cation.
Referring, therefore, to the formation of the sodium silicate in rotary tubular furnaces, the main reaction which takes place is the following: EQU n SiO.sub.2 + Na.sub.2 CO.sub.3 .fwdarw. n SiO.sub.2.Na.sub.2 O + CO.sub.2
where n may normally take up any value between 2 and 4, thereby obtaining a range of sodium silicates with distinct molar relationships, with specific characteristics and properties suitable for the applications to which they are destined.
In order that the melting should be perfect it is necessary to make the materials advance through the interior of the furnace at a uniform speed, without stoppages, forming a thin layer and in such a way that the time of stay of the materials within the furnace and the amount of such materials at any time present within the furance should be the indispensable minimum, so that the melting process should take its course in a progressive form between the entrance and the exit of the furnace. Thus, a series of successive stages, i.e., mixing stages, lumpiness stages, reaction and dissolution stages, liquid stage, and discharge stage are produced. During the process, at 750.degree.-800.degree. C. the lumpy state is begun between the silica and the carbonate, while from 800.degree. C. on begins the formation of the liquid stage (eutectic) which is made up of disilicate and quartz (silica), about 800.degree. C. the quartz grains are covered with a layer of metasilicate and disilicate, there taking place a great liberation of carbon dioxide, at which stage the reaction is very rapid and the carbon dioxide carries with it a part of the carbonate and the quartz, thereby forming a thin slag layer which floats on top of the liquid melted mass. At about 1,000.degree. C. the slag is dissolved (carbonate and quartz) in the liquid formed by metasilicate and disilicate, and, finally, about between 1,100.degree. C. and 1,200.degree. C. the gassing of carbon dioxyde ceases since all of the carbonate has reacted.
If the quartz has been previously calculated according to the indicated temperature, it will be totally dissolved within the liquid, and a silicate with the calculated molar ratio will be obtained. But it has to be taken into account that there exist diverse types of silicates which in the melted state possess different viscosities. Thus, it is obvious that if one had to deal with silicates whose melted mass is very thinly liquid, the flowing of the melted material out of the discharge exit of the furnace might be excessively rapid, thereby preventing the melting process from taking its course along the stages or phases hereinbefore mentioned, whereas, if, on the other hand, the melted mass or body is very viscous, the flowing of the melted material out of the exit of the furnace will be delayed, thereby increasing the thickness of the mass in the interior of the furnace. This will render difficult the intermixing of the materials and lead to an imperfect product.